Milk and milk products form an important part of the American diet and it is felt by some medical authorities that dietary fat is a factor in human atherosclerosis. For this and other reasons the development of basic information on the synthesis of the milk lipids is needed. Moreover, milk affords a particularly useful approach to the study of lipid metabolism at the level of a cell. We are attempting to understand how cell membranes are made, how they enable synthesis of milk components and how these components and membranes enter milk. We are also attempting to shed light on fat droplet formation (triglyceride accumulation) and secretion. A view is being maintained as to how findings could serve to improve milk and milk products as well as to unify the overall knowledge of lactation and the mammary epithelial cell. The research involves use of radioactive tracers and drugs to study metabolism of the mammary gland and electron microscopy to evaluate structural-functional aspects of cells. Recent findings indicate that fresh milk has the biochemical and cytological potential to synthesize milk fat droplets from glycerol and fatty acids.